Foam materials are typically classified as either rubber based foam materials or plastic based foam materials depending on the raw material. The applications for the different foam materials will also differ based on the different physical properties. Polyurethanes and polyethylenes of varying densities have been used as the raw material in the foaming process of manufacturing a plastic foam material. The manufactured plastic foam material may be cold-molding or hot-molding to form products having complicated shapes, may use simple foaming recipes, and may be easily colored. The foaming process and applications of products made therefrom are well known. The disadvantages of plastic foams however, are poor elasticity and poor slip resistance.
Rubber materials such as acrylonitrile butadiene rubber (NBR) have been used as the raw material in the process of manufacturing a rubber foam material. The manufactured rubber foam material provides weather-proof and physical properties different from plastic foams. However, compounding for rubber foam materials can become relatively complex. Additionally, crosslinking of the rubber compound must be controlled so that expansion of the rubber into the desired cellular structure may be achieved.
Foam materials may also be classified by the cellular structure. Open cells means the cells in the material are interconnected to provide openings to an external surface of the material. A closed cell foam material is one wherein the cells are surrounded by material membranes and are not interconnected to each other; the closed cell foam may have broken cells at the material surface, however, liquids could not pass from such a surface broken cell to the interior of the foam as it would with open cell foam.
U.S. Pat. No. 3,950,278 discloses a cross-linked polyolefin foam. The foam compound is comprised of ethylene and an uncrosslinkable propylene. To achieve the desired cellular content, the percent of propylene is restrained between 20 and 60%.
U.S. Pat. No. 4,421,867 discloses a cross-linked polyolefin foam. The foamed material comprises 80 to 20% polyethylene and 20 to 75% of a crystalline propylene copolymer containing 1 to 15 weight % ethylene, with the propylene copolymer having a greater than 50% crystallinity.